1. Field of the Invention
The present invention generally relates to a structure of a reducing agent container for storing therein liquid reducing agent to be used in an exhaust gas purification apparatus for the purpose of reducing and purifying nitrogen oxide (NOx) in an exhaust gas from an engine, and more particularly relates to a technique capable of exhibiting an exhaust gas purification function in a short time after an engine is started.
2. Description of the Related Art
As one typical catalyst purification system capable of removing NOx contained in an exhaust gas from an engine, Japanese Unexamined Patent Publication No. 2000-27627 (patent document 1) discloses and proposes an exhaust gas purification apparatus.
According to this exhaust gas purification apparatus, a reduction catalytic converter is disposed in an engine exhaust pipe, and a liquid reducing agent in response to the engine operating condition is injected to be supplied to the exhaust gas at a position upstream of the reduction catalytic converter, in order to produce a catalytic reduction reaction between NOx in the exhaust gas and the liquid reducing agent thereby purifying NOx into harmless components. In the reduction reaction, ammonia which has excellent reactivity with NOx is used. As the liquid reducing agent, urea aqueous solution which is hydrolyzed by heat of exhaust gas and vapor in the exhaust gas to generate ammonia is used.
Patent Document 1: Japanese Unexamined Patent Publication No. 2000-27627
In a cold district like the northernmost Hokkaido island, Japan, the outside air temperature in winter season becomes lower than a freezing point of the liquid reducing agent, and the liquid reducing agent stored in a reducing agent container is frozen in some cases. The liquid reducing agent starts freezing from an outer periphery thereof near the inner wall of the container that is in direct contact with the outside air, and the freeze gradually advances toward the central portion inside the container. At a bottom of the reducing agent container, there are detectors such as a water level indicator and a densitometer which respectively detect remaining amount and concentration of the liquid reducing agent, and a suction port of the liquid reducing agent. Therefore, there is an adverse possibility that, if the liquid reducing agent is frozen even slightly, the remaining amount and concentration of the liquid reducing agent may be undetectable and the liquid reducing agent can not be supplied to the reduction catalytic converter. For this reason, the reducing agent container is provided with a heat exchanger which circulates engine coolant to exchange heat between the engine coolant and the liquid reducing agent. If the engine is stopped for a long time, however, since the coolant temperature is low immediately after the engine is started, it takes a certain amount of time to unfreeze the frozen liquid reducing agent.